Superalloy powders are typically produced by inert atomization processes such as argon atomization, vacuum atomization, rotating electrode process and rotary disk atomization. Water atomization processes are not generally acceptable due to the formation of a heavy surface oxide produced by a chemical reaction of the form: xMe+yH.sub.2 O=Me.sub.x O.sub.y +yH.sub.2. Reactive elements (Si, Al, Ti, Cr, Mn) are oxidized and are difficult to reduce in subsequent processing. Since oxides are detrimental to the product's mechanical properties, inert atomization processes (oxygen &lt;200 ppm) are used.
Unfortunately, inert atomization processes produce spherical powders which are not satisfactory for standard die compaction processes. These powders require special consolidation practices such as HIP (Hot Isostalic Pressing), Cercon, CAP (consolidated at pressure), etc. which are rather expensive. Due to costs of gas atomization and consolidation, the use of powder metallurgy for superalloy production has been limited to aerospace applications where the expense is justified.
There is a need for a superalloy powder that can be die compacted using existing technology. Such a powder should have an irregular shape, small average particle size and low oxygen content (&gt;200 ppm). Water atomization can produce the irregular powder, but the oxygen content is too large. If the oxides can be removed in a cost effective process, these powders would be commercially attractive. In the steel industry, some strides are being made to satisfy these requirements. Stainless steel powders (304L, 316L, 410 and 430 grades) containing chromium and/or manganese are available and are being used to lower the cost and improve the hardenability of a finished product. These powders are produced by water atomization under conditions that minimize the oxygen level (oxygen &gt;1500 ppm). Some of these parameters are an inert purge of the atomization chamber, lower silicon heats, use of soft water (low calcium), and minimizing liquid turbulence during melting to reduce slag impurities. Further, during processing a high temperature sintering operation is used with careful control of dew point and carbon reduction to remove any oxides. In another related process (QMP), tool steels are made from water atomized powders by producing a high carbon heat. During the sintering operation a self generated CO-CO.sub.2 atmosphere reduces the oxygen content.
The ultimate aim is to produce a low oxygen containing product or powder by removing the tenacious surface oxide from lower cost water atomized powders. One promising method for accomplishing this goal requires pickling the powder. Difficulties arise in optimizing the pickling procedure including the selection of the baths and their utilization.
Other researchers have demonstrated the favorable effects of pickling powders in various alloy systems. In U.S. Pat. No. 2,638,424 a process is disclosed for processing aluminum and magnesium powders to remove detrimental oxide and nitride films. The powders are treated with nitric acid in a continuous process. In U.S. Pat. No. 4,477,296, noble metal powders (Au, Pd, Ag, Pt and/or alloys) or base metal powders (Cu, Ni, Sn, Al, Sb, Ti, V, Cr, Mg, Fe, Co, Zn, Cd, Rh) are surface treated to remove undesirable oxides. The key application of this invention is in the manufacture of multilayer capacitors. The described invention consists of: (a) treating the surface with an aqueous solution of a reducing agent for the oxide; (b) washing the powders with an aqueous solution to a pH of 5.5-7.0 and (c) drying the powders.
In a related topic, U.S. Pat. No. 4,566,939 describes a method for removal of undesirable oxides from aluminum or titanium containing nickel-iron-base or nickel-base alloys prior to brazing or diffusion bonding. The workpiece is heat treated above 1800.degree. F. (982.degree. C.) to form an Al/Ti rich oxide. This oxide is removed using a strong alkaline solution and/or molten salt bath. It is reported that the alkaline solution is preferred over acid solutions for removing surface oxides because they do not etch or attack the base metal or remove the depleted Al/Ti layer beneath the surface oxide.
Another related area involves the application of a sintering activator during the pickling sequence. There are several patents pertaining to the use of boron as a sintering activator. U.S. Pat. No. 3,704,508 deals with the well known CAP process where boric acid is used as a sintering activator. U.S. Pat. No. 4,407,775 teaches the use of lithium tetraborate additions to powders as a sintering activator. U.S. Pat. No. 4,113,480 deals with injection molding using a boric acid-glycerin system for mold release and activated sintering. Lastly, assignee's U.S. Pat. No. 4,626,406 deals with the use of boron containing activators in P/M slurry extrusions.